Method for pulp bleaching

ABSTRACT

A process for pulp bleaching is disclosed wherein an amino acid phosphonic acid is used together with a bleaching agent, pH regulants and pH buffers for pulp treatment in an aqueous medium. The bleaching agent can be represented by oxidative and reductive bleaching agents. The oxidative bleach treatment is conducted in alkaline medium whereas the reductive treatment is conducted in mildly acid medium.

This invention pertains to a method of pulp bleaching wherein pulp isbleached in an aqueous medium by means of an oxidative or reducingbleaching agent in the presence of an amino acid alkyl phosphonic acidor a neutralized version thereof. In particular, the pulp is treatedwith an additive level of the bleaching agent in the presence of anadditive level of a narrowly defined amino acid alkyl phosphonic acid ata temperature in the range from about 30 to 100° C. for a period of 1minute to 8 hours. The method was found to yield particularly beneficialresults in the event the bleaching agent is an oxidative bleach wherebythe treatment is applied at a pH in the range of from 8-13.

Pulp bleaching technology broadly has been around for a long time andhas found considerable commercial application. Alkyl phosphonic acidshave been used in the very domain with mitigated success. Actually, themost preferred phosphonic acid compounds for use in pulp bleachingtechnology is said to be diethylene triamino penta(methylene phosphonicacid) (DTPMP). There was a considerable and standing desire to developimproved bleaching approaches.

US 2008/0115692 pertains to a method and compositions for improvingproperties of pulp produced in alkaline chemical pulping processeswherein at least one conventional phosphonate is used. Chinese patentapplication CN 2008-10106832 relates to a method for preparingstabilizer composition for hydrogen peroxide used in paper pulpbleaching. The alkaline stabilizer composition containing EDTA,polyacrylic acid, nitrilotriacetic acid, hydroxyethylidene diphosphonicacid and sodium hydroxide can serve to reduce the decomposition ofhydrogen peroxide to below 20%. The use of solutions ofpoly(α-hydroxyacrylic acid) salt and diethylene triamine penta-acetatehaving a pH of 3-8 as stabilizers in the hydrogen peroxide bleaching offibre and paper pulp products is described in JP 1993-182234.

WO 2009/092738 and EP 2 082 991 pertain to the use of aqueous mediumunder substantial exclusion of metal ion interference. A selectedphosphonic acid is used for the effective immobilization of metal ions.WO 99/46441 concerns a method for bleaching paper pulp with the aid ofper oxidized oxidants consisting, inter alia, by pretreating the paperpulp under alkaline conditions by means of an aspartic acid chelatingagent. U.S. Pat. No. 5,759,440 describes a method for stabilizing anaqueous hydrogen peroxide solution by means of a pyrophosphate salt andan aminopolycarboxylic acid. The stabilized solution can be used forpulp bleaching.

WO 2008/086937 discloses a process for pulp bleaching comprising addingan amino phosphonate in combination with an oxidative bleaching agent.WO 00/68396 discloses a pulp bleaching agent, where the activeingredient is a XA protein type xylanase. U.S. Pat. No. 4,372,811disclose a process for pulp bleaching, where one or more aromaticdiamines are added to inhibit degradation of carbohydrates in the pulp.

The search for novel technologies as e.g. shown by the art recitationshas not yielded superior technologies capable of meeting expectationsand desires. Indeed significant hurdles inclusive of active peroxidelosses in oxidative pulp bleaching and effective heavy metal ion controlremain obstacles, can be fairly expensive and can yield reducedperformance. In addition, leading phosphonates currently used e.g. DTPMPtend to exhibit reduced bleaching activity at higher pH e.g. above 11and contribute to undue peroxide decomposition.

It is a major object of this invention to generate superior pulpbleaching technology. It is another object of this invention to providesuperior oxidative pulp bleaching technology characterized, inparticular, by significantly reduced active peroxide losses. Stillanother object of this invention contemplates generating pulp bleachingtechnology with a significantly improved metal ion control andimmobilization. Yet another object of this invention aims at securingpulp bleaching technology capable of yielding pulp products havingmarkedly superior physical properties as compared to what can beachieved from using the art technology.

The term “percent” or “%” as used throughout this application stands,unless defined differently, for “percent by weight” or “% by weight”.The terms “phosphonic acid” and “phosphonate” are also usedinterchangeably depending, of course, upon medium prevailingalkalinity/acidity conditions. The term “ppm” stands for “parts permillion”. The terms “phosphonic acid” and “phosphonate” are usedinterchangeably depending upon medium prevailing alkalinity/acidityconditions. The terms “pulp” and “pulp consistency” are usedinterchangeably. Unless defined differently, pH values are measured at25° C. on the reaction medium as such. The term amino acid stands foramino acids in their D, L and DL forms as well as mixtures of the D andL forms. The term “aromatic” hydrocarbon radical or moiety preferablyincludes aryl and heteroaryl groups.

The above and other objects can now be met by using a novel bleachingarrangement whereby the bleaching treatment is carried out in thepresence of an amino acid alkyl phosphonic acid. In more detail theinventive method herein concerns pulp bleaching comprising the steps of:

treating an aqueous medium containing from 1% to 40% by weight of thepulp, expressed on the basis of the water in the aqueous medium (100%);

adding a bleaching agent, selected from oxidative bleaches and reducingbleaches, in an amount from 0.5% to 10% by weight, expressed on thebasis of the dry pulp (100%), and bleaching additives selected from pHregulants and buffer components to conduct the oxidative bleachingtreatment at a pH of from 8 to 13 and the reductive bleaching treatmentat a pH of from 2 to 6.5;

adding an amino acid alkyl phosphonic acid or a salt, i.e. a neutralizedversion, thereof having a formula selected from:

A¹—(B)_(y);

and

A²—(B)_(y);

wherein A¹ and A² have the formula:

A¹=HOOC—A—NH₂;

and

A²=HOOC—C (NH₂) (R) (R′)

wherein B is an alkylphosphonic acid moiety having from 1 to 6 carbonatoms in the alkyl group and y is an integer of from 1 to 10; A isindependently selected from C₂-C₂₀ linear, branched, cyclic and aromatichydrocarbon radicals, optionally substituted by one or more C₁-C₁₂linear, branched, cyclic and/or aromatic hydrocarbon groups, whichradicals and/or which groups are optionally substituted by one or moreOH, COOH and/or NH₂ moieties; R and R′ are independently selected fromC₁-C₂₀ linear, branched, cyclic and aromatic hydrocarbon radicals,optionally substituted by one or more C₁-C₁₂ linear, branched, cyclicand/or aromatic groups, NH₂ and/or COOH, and one of R or R′ can behydrogen;whereby the neutralizing agent to obtain the salt is preferably selectedfrom ammonia, alkali hydroxide, earth-alkali hydroxide and amine wherebythe amine preferably has the general formula

(X)_(b)[N(W)(H)_(2-b)]_(z)

wherein X is selected from C₁-C₂₀₀₀₀₀ linear, branched, cyclic oraromatic hydrocarbon radicals, optionally substituted by one or moreC₁-C₁₂ linear, branched, cyclic or aromatic groups which radicals and/orwhich groups are optionally substituted by OH, COOH, COOG, F, Br, Cl, I,OG, SO₃H, SO₃G and/or SG moieties; H; [V—N(H)]_(x)—H ; [V—N(Y)]_(n)—V;[V—O]_(x)—V; wherein V is selected from: C₂₋₅₀ linear, branched, cyclicor aromatic hydrocarbon radicals, optionally substituted by one or moreC₁-12 linear, branched, cyclic or aromatic groups, which radicals and/orgroups are optionally substituted by OH, COOH, COOR″, F/Br/Cl/I, OR″,SO₃H, SO₃R″ and/or SR″ moieties, wherein R″ is a C₁₋₁₂ linear, branched,cyclic or aromatic hydrocarbon radical, wherein G is selected fromC₁-C₂₀₀₀₀₀ linear, branched, cyclic or aromatic hydrocarbon radicals,optionally substituted by one or more C₁-C₁₂ linear, branched, cyclic oraromatic groups, which radicals and/or which groups are optionallysubstituted by OH, COOH, COOR″, F, Br, Cl, I, OR″, SO₃H, SO₃R″ and/orSR″ moieties; H; [V—N(H)]_(n)—H; [V—N(Y)]_(n)—V or [V—O]_(x)—V; whereinY is H, [V—N(H)]_(n)—H or [V—N(H)]_(n)—V and x is an integer from1-50000, n is an integer from 0 to 50000; z is from 0-200000 whereby zis equal to or smaller than the number of carbon atoms in X, and b is 0or 1; z=1 when b=0; and X is [V—N(H)]_(x)—H or [V—N(Y)]_(n)—V when z=0and b=1;with the proviso of excluding:compounds wherein R and/or R′ are electron rich moieties containing, atleast, one lone pair of electrons, which moiety is directly attached toan aromatic moiety by a covalent bond; or aromatics wherein at least oneof the carbon atoms has been substituted by a heteroatom; and compounds,in the event R is —C(Z)(R′″)(R″″) and R′, R′″ and R″″are hydrogenwherein Z is an electron withdrawing group selected from NO₂, CN, COOH,SO₃H, OH and halogen, andwith the further proviso that when:

A² is L-lysine, at least one L-lysine amino radical carries 2 (two)alkyl phosphonic acid moieties; and when

A² is L-glutamic acid, the term glutamic acid phosphonate represents acombination of from 50-90% by weight pyrrolidone carboxylic acidN-methylene phosphonic acid and from 10-50% by weight of L-glutamic aciddiphosphonic acid, expressed on the basis of the reaction products;

said amino acid alkyl phosphonic acid compound being added in a level offrom 0.01% to 6% by weight, expressed on the level of the dry pulp(100%);said bleaching treatment being conducted at a temperature from 30° C. to100° C. for a period of from 1 minutes to 8 hours.

A first essential amino acid alkyl phosphonic acid for use in the methodof this invention can be represented by the formula:

A¹—(B)_(y)

wherein A¹ has the formula

HOOC—A—NH₂

wherein A is independently selected from C₂-C₂₀ linear, branched, cyclicand aromatic hydrocarbon radicals, optionally substituted by one or moreC₁-C₁₂ linear, branched, cyclic and/or aromatic hydrocarbon groups,which radicals and/or which groups are optionally substituted by one ormore OH, COOH and/or NH₂ moieties. In a preferred execution, A isrepresented by a C₂-C₁₆ linear hydrocarbon chain, optionally, andpreferably, substituted by 1 to 3 NH₂ moieties. The selection of anynumber of carbon atoms in the hydrocarbon chain can constitute adesirable execution depending upon the choice of additional optionalgroups and/or optional moieties. The actual determination of preferredcombinations is a routine measure, well known in the domain of thetechnology.

A second essential amino acid alkyl phosphonic acid for use in themethod of this invention can be represented by the formula:

A²—(B)_(y)

wherein A² has the formula

HOOC—C (NH₂) (R) (R′)

wherein R and R′ are independently selected from C₁-C₂₀ linear,branched, cyclic and aromatic hydrocarbon radicals, optionallysubstituted by one or more C₁-C₁₂ linear, branched, cyclic and/oraromatic hydrocarbon groups, which radicals and/or groups are optionallysubstituted by one or more OH, NH₂ and/or COOH moieties, and one of R orR′ can be hydrogen.

In a preferred execution of the method herein, the amino acid in thephosphonate inhibitor A² can be represented by D,L-alanine wherein y is2, L-alanine wherein y is 2, L-lysine wherein y is in the range of from2 to 4, L-phenylalanine wherein y is 2, L-arginine wherein y is in therange of from 2-6, L-threonine wherein y is 2, L-methionine wherein y is2, L-cysteine wherein y is 2 and L-glutamic acid wherein y is 1 to 2.

It was found that the L-glutamic acid alkylene phosphonic acid compoundas such is, because of insufficient performance and stability, notsuitable for use in the method of this invention. Depending upon theformation reaction conditions, the L-glutamic acid alkylene phosphonicacid resulting from the methylenephosphonation of L-glutamic acid can berepresented by a substantially binary mixture containing, based on themixture (100%), a majority of a mono-methylene phosphonic acid derivedfrom a carboxylic acid substituted pyrrolidone and a relatively smallerlevel of a dimethylene phosphonic acid glutamic acid compound. It wasfound that, in one beneficial embodiment the reaction product frequentlycontains from 50% to 90% of the pyrrolidone carboxylic acid N-methylenephosphonic acid scale inhibitor and from 10% to % of the L-glutamic acidbis(alkylene phosphonic acid) compound. The sum of the diphosphonate andmonophosphonate inhibitors formed during the reaction frequently exceeds80%, based on the glutamic acid starting material. The binary mixturecan also be prepared by admixing the individual, separately prepared,phosphonic acid compounds. In another preferred execution, the L-lysinecarrying one alkylene phosphonic acid group attached to amino radical(s)represents not more than 20 molar % of the sum of the L-lysine carryingone and two alkylene phosphonic acid groups attached to aminoradical(s). In another preferred execution, the L-lysine alkylenephosphonic acid is represented by a mixture of L-lysine carrying twoalkylene phosphonic acid groups attached to (individual) aminoradical(s) (lysine di) and L-lysine carrying four alkylene phosphonicacid groups (lysine tetra) whereby the weight ratio of lysine tetra tolysine di is in the range of from 9:1 to 1:1, even more preferred 7:2 to4:2.

Preferred aminoacids in the A² phosphonate inhibitors include7-aminoheptanoic acid, wherein x is 2,6-aminohexanoic acid, wherein x is2,5-aminopentanoic acid, wherein x is 2,4-amino butyric acid, wherein xis 2 and β-alanine wherein x is 2. Preferred aminoacids in thephosphonate inhibitors A ¹ can be prepared beneficially starting fromlactams or other conventionally known materials; 7-aminoheptanoic acidcan be used instead of 2-azacyclooctanone to make the correspondingdiphosphonate. The preferred aminoacid starting materials areillustrated in the examples hereinafter. In short, a mixture ofstoichiometric proportions of the starting material aminoacid (1 mole),phosphorous acid (2 moles), aqueous hydrochloric acid (1.2 moles) isheated under stirring to 100 ° C., the formaldehyde (2 moles) is thengradually added over a period of 120-140 minutes at a temperature in therange of from 100-120 ° C. The reaction mixture is thereafter kept at105-115 ° C. for an additional 60-100 minutes. It is understood that thestoichiometric proportions of the starting materials can be varied tomeet the desired degree of phosphonic acid substitution by reaction withthe available N—H functions.

In another preferred execution herein, the amino acid phosphonate foruse in the method of this invention can be represented by selectedcombinations of the amino acid polyphosphonates in combination with apolyphosphonic acid selected from the group of: (a) amino(poly)alkylenepolyphosphonic acids wherein the alkylene moiety contains from 1 to 20carbon atoms; (b) hydroxyalkylene polyphosphonic acids wherein thealkylene moiety contains from 2 to 50 carbon atoms; and (c) phosphonoalkane polycarboxylic acids wherein the alkane moiety is in straightchain configuration containing from 3 to 12 carbon atoms. Actuallypreferred are: aminoalkylene polyphosphonic acids having from 1 to 12carbon atoms in the alkylene moiety; hydroxyalkylene phosphonic acidscontaining from 2 to 12 carbon atoms in the alkylene moiety and twophosphonic acid groups; whereas phosphono alkane polycarboxylic acidshave a straight chain alkane configuration having from 4 to 8 carbonatoms and wherein the molar ratio of phosphonic acid radical tocarboxylic acid radical is in the range of from 1:2 to 1:4. Particularlypreferred are polyphosphonic acids having from 2 to 8 phosphonic acidgroups. Individually preferred species were found to include thefollowing: aminotri(methylene phosphonic acid) and its N-oxide;1-hydroxyethylene(1,1-diphosphonic acid); ethylenediaminetetra(methylene phosphonic acid); diethylene triaminepenta(methylenephosphonic acid); hexamethylene diamine tetra(methylenephosphonic acid); hydroxyethyl aminobis(methylene phosphonic acid);N,N′-bis (3-aminopropyl)-ethylenediamine hexa(methylene phosphonicacid); and butane-2-phosphono-1,2,4-tricarboxylic acid.

The weight ratio of amino acid phosphonate to phosphonic acid is in therange of from 98:2 to 25:75, preferably from 90:10 to 50:50.

A² can be represented by α-amino acids including specific natural aminoacids such as e.g. occurring in animal species. Amino acids generallyare the building blocks of proteins. There are over forty known aminoacids about twenty of which are actually contained in e.g. animaltissue. Amino acids can be made by hydrolysis starting from proteins, byenzymatic fermentation and/or by chemical synthesis. This domain of thetechnology is eminently well known and all the individual technologiesare abundantly documented in the literature. Suitable amino acids can beused in their D, D, L, and L forms as well as mixtures of the D and Lforms. Preferred α-amino acids for use in the phosphonate inhibitorsinclude: D,L-alanine; L-alanine; L-phenylalanine; L-lysine; L-arginine;L-methionine; L-cysteine; L-threonine; and L-glutamic acid.

Specific Amino Acids are Excluded as Follows:

1. α-aminoacids wherein R and/or R′ comprise electron rich moietiesdirectly attached to an aromatic moiety. As an example, the reaction ofL-tyrosine (1 eq.) (R═ p-OH-Phenyl-CH₂; R′═H) with H₃PO₃ (2 eq.) andformaldehyde (2.2 eq.) in the presence of HCl (1.5 moles) between 108and 112° C. does not lead to the corresponding bis(methylene phosphonicacid). Indeed, ³¹P NMR analysis only shows signals for the startingphosphorous acid with traces of phosphoric acid. A water insolubleproduct is obtained; it is believed to be due to the reaction offormaldehyde with tyrosine resulting in the formation of methylenebridges between aromatic moieties;

2. α-aminoacids wherein R and/or R′ comprise aromatics wherein at leastone carbon atom has been substituted by a heteroatom. For example, thereaction of L-tryptophan (1 eq.) with H₃PO₃ (2 eq.) and formaldehyde(2.2 eq.) in the presence of HCl (2.5 moles) between 107 and 111° C.does not lead to the corresponding bis(methylene phosphonic acid). ³¹PNMR analysis only shows signals for the starting phosphorous acid withtraces of phosphoric acid. A water insoluble product is obtained; it isbelieved to be due to the reaction of formaldehyde with tryptophanresulting in the formation of methylene bridges between aromaticmoieties; and

3. α-aminoacids wherein in the event R is —C(Z)(R′″)(R″″) and R′, R′″and R″″ are hydrogen wherein Z is an electron withdrawing group selectedfrom NO₂, CN, COOH, SO₃H, OH and halogen. As an example, the reaction ofL-aspartic acid (1 eq.) (Z═COOH) with H₃PO₃ (2 eq.) and formaldehyde(2.2 eq.) in the presence of HCl (1.5 moles) between 110 and 115° C.leads to a complex product mixture including: fumaric acid;imino-bis(methylene phosphonic acid); aminotri(methylene phosphonicacid) (ATMP) and L-aspartic acid bis(methylene phosphonic acid). Thelatter product has been shown by ³¹P NMR to decompose under the reactionconditions into fumaric acid and imino bis(methylene phosphonic acid)which is itself converted into ATMP. In another example, the reaction ofL-serine (1 eq.) (Z═OH) with H₃PO₃ (2 eq.) and formaldehyde (2.2 eq.) inthe presence of HCl (1.5 moles) between 107 and 112 ° C. leads to acomplex product mixture including amino tri(methylene phosphonic acid)(ATMP) and phosphorous acid. ³¹P NMR does not show signals correspondingto the L-serine mono- or di-phosphonates. It is believed that theL-serine phosphonates are unstable and decompose, under the reactionconditions, ultimately leading to ATMP.

Specific α-aminoacids not suitable for use within the claimed technologyare: tyrosine; tryptophan; asparagine; aspartic acid; and serine.

The amino acid alkylphosphonates for use in the inventive method can beprepared by reacting one or more of the available N—H functions of theamino acid with phosphorous acid and formaldehyde, in the presence ofhydrochloric acid, in aqueous medium having a pH of generally less than4 by heating that reaction mixture, at a temperature of usually greaterthan 70° C. for a sufficient time to complete the reaction. This kind ofreaction is conventional and well-known in the domain of the technologyand examples of the novel phosphonate compounds have been synthesized,as described below, via the hydrochloric acid route.

In a preferred method, the aminoacid phosphonates can be made undersubstantial exclusion of hydrohalogenic acid and correspondingby-products and intermediates. Specifically, the aminoacid phosphonatescan be manufactured in presence of not more than 0.4%, preferably lessthan 2000 ppm, of hydrohalogenic acid, expressed in relation to thephosphorous acid component (100%) by reacting:

(a) phosphorous acid;

(b) an aminoacid; and

(c) a formaldehyde:

in reactant ratios of (a):(b) of from 0.05:1 to 2:1; (c):(b) of from0.05:1 to 5:1; and (c):(a) of from 5:1 to 0.25:1;wherein (a) and (c) stand for the number of moles to be used and (b)represents the number of moles multiplied by the number of N—H functionsin the amine, in the presence of an acid catalyst having a pKa equal orinferior to 3.1, said catalyst being homogeneous with respect to thereaction medium and being used in reactant ratios as follows:(b):(d) of from 40:1 to 1:5;wherein (d) stands for the number of moles of catalyst multiplied by thenumber of available protons per mole of catalyst. The aminoacidphosphonates formed can be recovered in a manner known per sé. Thereaction can also be conducted similarly to the homogeneous catalystparameter selection in the presence of a heterogeneous catalyst selectedfrom e.g. solid acidic metal oxides as such or deposited onto a carrier;cation exchange resins carrying sulfonic or carboxylic Broensted acidgroups; acid catalysts derived from the interaction of a solid supporthaving a lone pair of electrons onto which is deposited an organicBroensted catalyst; the interaction of such support onto which isdeposited a compound having a Lewis acid site; heterogeneous solidsfunctionalized by chemical Broensted grafting and heterogeneousphosphorus and silicon containing polyacids.

Pulp is a dry fibrous material prepared by chemically or mechanicallyseparating fibers from wood, fiber crops or waste paper. There are anumber of different processes which can be used to separate wood fibers.These processes are very well known in the domain of the technology. Thepulp designation can relate to the process used, e.g. known species are:mechanical pulp; thermomechanical pulp; chemithermomechanical pulp;chemical pulp; and recycled pulp, also known as deinked pulp. The pulpso available can be bleached to produce e.g. a white paper product. Oneobjective of bleaching is to improve the brightness and the cleanlinessof the pulp product. The bleaching agent can be selected from oxidativebleaches and reductive bleaches which bleaches are used in a level offrom 0.5 to 10%, preferably from 1 to 6%, in particular from 2 to 5%,expressed on the basis of the dry pulp (100%). Well known examples ofbleaching agents suitable for use herein include dithionite,boronhydride, chlorine dioxide, peroxomonosulferic acid (H₂SO₅),formamidine sulfinic acid, activated acid peroxide e.g. molybdateperoxide; oxygen, peroxide reinforced oxygen, hydrogen peroxide,peracids such as peracetic acid and ozone. In general, the oxidativebleaching agent shall not react with the aminoacid phosphonate. Chlorineand hypochlorite can react with the amino acid phosphonate and can notbe used in the oxidative bleaching step herein. The oxidative bleachingtreatment is generally conducted at a pH in the range of from 8-13,preferably 8-12, in particular 8.5-10.5. The reductive bleachingtreatment is generally carried out at a pH in the range of from 2-6.5,preferably 3-6.5, in particular 4-6.5. The bleaching step requires,evidently, the presence of suitable pH regulants, buffer components andoptionally additives conventional and well known in the domain of thetechnology. The pH can during the oxidative approach be established withthe aid of alkaline materials, in particular alkaline hydroxidesincluding sodium and potassium hydroxide, alkaline earth hydroxidesincluding magnesium and calcium hydroxide, silica buffers and alsoMgSO₄·7H₂O, Epsom salt. The use of regulant combinations of magnesiumhydroxides and magnesium salts was found to be beneficial in that it candeliver better alkalinity reserve, compared to e.g. alkali hydroxidealone, under comparable level conditions. As an example, the pH can beadjusted during the oxidative bleaching with the aid of sodium hydroxidewhereas sulfuric acid can be used for pH control preparatory to thereductive bleaching step.

The pulp is used during the bleaching treatment in a level of from 1 to40%, preferably from 3 to 30%, in one particular execution from 4 to 25%expressed on the basis of the aqueous medium. These ranges are fairlystandard in the relevant technical domain. These ranges can be, andfrequently are, slightly different depending upon which bleaching step,oxidative or reductive, is used. The pulp consistency is usually higherfor the oxidative bleaching step than for the reducing bleaching stepalthough the respective levels are within the ranges recited herein.

The inventive bleaching treatment is conducted for a period of from 1minute to 8 hours, preferably from 8 minutes to 6 hours, in particularfrom 20 minutes to 2 hours at a temperature of from 30 to 100° C.,preferably from 40 to 90 ° C., in particular from 45 to 80° C. In theprocess of the invention the sequence of adding the amino acid alkylphosphonic acid component and the bleaching agent is not fixed. In apreferred embodiment the amino acid alkyl phosphonic acid component isadded first. In a further embodiment both components are simultaneouslyadded. Of course it is also possible to add the pulp to the components.

The method herein generally has been practiced extensively for a longtime. A summary description can serve as a brief reminder of the knownstate of the art. The bleaching method herein can embody multiplevariations whereby the most common approaches embody single or multiplesubsequent stages. As an example, a one-stage bleaching processcomprises adding the pulp and the chelant before the pulp thickenerfollowed by a steam mixer where the bleaching agent, frequently a bleachsolution, is added. The bleaching can occur in a bleach tower followedby neutralizing before transferring the bleached pulp to the papermachine. As an example of another approach, a two-stage bleachingprocess is arranged whereby an oxidative bleaching step is followed by areductive bleaching step. Actually, the oxidative step can be identicalto the one-stage process above up to the neutralizing stage. At thattime, the reductive bleach is added together with the bleached pulporiginating from the first stage, and possibly additional chelant, intoa second bleaching tower, e.g. an up flow tower. The bleached pulp isusually kept in a stock chest and can from there be transferred to thepaper machine, possibly via a pulp thickener to generate the right pulpconsistency. These technologies are well known and any method variationis standard practice.

The potential benefits attached to the application of the technology ofthis invention is illustrated with the aid of comparative showings asfollows.

Performance of the amino acid phosphonates have been compared withtraditional products used for the stabilization of the oxidative bleachused in conditions similar to the paper making processes. Products usedin this comparison are the diethylene triamino penta-(methylenecarboxylic acid) (DTPA); diethylene triamino penta-(methylene phosphonicacid) (Dequest® 2066) and the lysine tetra-(methylene phosphonic acid)ethanolamine salt (Lysine tetraphph).

A test is conducted as follows: In a three neck flask equipped with amechanical stirrer, a constant pressure dropping funnel and a tubingconnected to a volumetric measuring devise are placed 0.2 g of the testproduct; 6 g of a 32% w/w magnesium hydroxide aqueous slurry; 3 g ofsodium silicate and 10 g of a 1% iron sulfate hepta hydrate in water.Demineralized water is added up to a total weight of 150 g. The mixtureis heated to 60° C. under stirring before addition of 16.7 g of a 42%aqueous hydrogen peroxide solution. Volumes of oxygen gas formed afteraddition of the hydrogen peroxide are recorded on a time basis. Table 1gives the experimental results where time is expressed in minutes (min)and oxygen volumes in milliliters (ml).

TABLE 1 Volumes of oxygen emissions during the 120 min experiment.Example 1 Comparative Oxygen volumes Comparative Ex. 1 Ex. 2 Time(ml)for Lysine Oxygen volumes (ml) Oxygen volumes (min) tetraphph forDequest 2066 (ml) for DTPA 0 0 0 0 2 5 15 5 4 25 60 40 6 45 100 85 8 70145 130 10 100 195 185 12 125 255 240 14 160 315 295 16 180 370 355 18200 420 410 20 225 475 475 22 255 530 530 24 280 580 585 26 305 635 64028 335 685 685 30 360 735 735 32 385 780 780 34 420 825 830 36 435 870870 38 460 910 915 40 485 955 965 60 750 1310 1315 75 875 1515 1500 901015 1665 1655 120 1220 1910 1845

The volume of oxygen gas formed after addition of the hydrogen peroxideis significantly by reduced in the inventive method.

1. A method for pulp bleaching comprising the steps of: providing anaqueous medium containing from 1% to 40% by weight of the pulp,expressed on the basis of the water in the aqueous medium (100%); addingan amino acid alkyl phosphonic acid or a salt thereof having a formulaselected from:A¹—(B)_(y); andA²—(B)_(y); wherein A¹ and A² have the formula:A¹=HOOC—A—NH₂; andA²=HOOC—C(NH₂)(R)(R′) wherein B is an alkylphosphonic acid moiety havingfrom 1 to 6 carbon atoms in the alkyl group and y is an integer of from1 to 10; A is independently selected from C₂-C₂₀ linear, branched,cyclic and aromatic hydrocarbon radicals, optionally substituted by oneor more C₁-C₁₂ linear, branched, cyclic and/or aromatic hydrocarbongroups, which radicals and/or which groups are optionally substituted byone or more OH, COOH and/or NH₂ moieties; R and R′ are independentlyselected from C₁-C₂₀ linear, branched, cyclic and aromatic hydrocarbonradicals, optionally substituted by one or more C₁-C₁₂ linear, branched,cyclic and/or aromatic groups, such as phenyl, NH₂, NH—(C═NH)NH₂, OH,SH, SCH₃, NO₂, CN, SO₃H, halogen, CONH₂ and/or COOH, and one of R or R′can be hydrogen; with the proviso of excluding: compounds wherein Rand/or R′ comprise electron rich moieties containing, at least, one lonepair of electrons, which moiety is directly attached to an aromaticmoiety by a covalent bond; or aromatics wherein at least one of thecarbon atoms has been substituted by a heteroatom; and compounds, in theevent R is —C(Z)(R′″)(R″″) and R′, R″″ and R″″ are hydrogen wherein Z isan electron withdrawing group selected from NO₂, CN, COOH, CONH₂, SO₃H,OH and halogen, and with the further proviso that when: A² is L-lysine,at least one L-lysine amino radical carries 2 (two) alkyl phosphonicacid moieties; and when A² is L-glutamic acid, the term glutamic acidphosphonate represents a combination of from 50-90% by weightpyrrolidone carboxylic acid N-methylene phosphonic acid and from 10-50%by weight of L-glutamic acid diphosphonic acid; said aminoacidalkylphosphonic acid compound being added in a level of from 0.01% to 6%by weight, expressed on the level of the dry pulp (100%); adding ableaching agent, selected from oxidative bleaches and reducing bleaches,in an amount from 0.5% to 10% by weight, expressed on the basis of thedry pulp (100%), and bleaching additives selected from pH regulants andbuffer components to conduct the oxidative bleaching treatment at a pHof from 8 to 13 and the reductive bleaching treatment at a pH of from 2to 6.5; and conducting a bleaching treatment at a temperature from 30°C. to 100° C. for a period of from 1 minutes to 8 hours.
 2. The methodin accordance with claim 1, wherein the salt of the amino acid alkylphosphonic acid is generated with a neutralizing agent selected fromammonia, alkali hydroxides, earth-alkali hydroxides and amines.
 3. Themethod in accordance with claim 2, wherein the neutralizing agent isselected from amines having the general formula(X)_(b)[N(W)(H)_(2-b)]_(z) wherein X is selected from C₁-C₂₀₀₀₀₀ linear,branched, cyclic or aromatic hydrocarbon radicals, optionallysubstituted by one or more C₁-C₁₂ linear, branched, cyclic or aromaticgroups which radicals and/or which groups are optionally substituted byOH, COOH, COOG, F, Br, Cl, I, OG, SO₃H, SO₃G and/or SG moieties; H;[V—N(H)]_(x)—H ; [V—N(Y)]—V; [V—O]_(x)—V; wherein V is selected from:C₂₋₅₀ linear, branched, cyclic or aromatic hydrocarbon radicals,optionally substituted by one or more C₁₋₁₂ linear, branched, cyclic oraromatic groups, which radicals and/or groups are optionally substitutedby OH, COOH, COOR″, F/Br/Cl/I, OR″, SO₃H, SO₃R″ and/or SR″ moieties,wherein R″ is a C₁₋₁₂ linear, branched, cyclic or aromatic hydrocarbonradical, wherein G is selected from C1-C₂₀₀₀₀₀ linear, branched, cyclicor aromatic hydrocarbon radicals, optionally substituted by one or moreC₁-C₁₂ linear, branched, cyclic or aromatic groups, which radicalsand/or which groups are optionally substituted by OH, COOH, COOR″, F,Br, Cl, I, OR″, SO₃H, SO₃R″ and/or SR″ moieties; H; [V—N(H)]_(n)—H;[V—N(Y)]_(n)—V or [V—O]_(x)—V; wherein Y is H, [V—N(H)]_(n)—H or[V—N(H)]_(n)—V and x is an integer from 1-50000, n is an integer from 0to 50000; z is from 0-200000 whereby z is equal to or smaller than thenumber of carbon atoms in X, and b is 0 or 1; z=1 when b=0; and X is[V—N(H)]_(x)—H or [V—N(Y)]_(n)—V when z=0 and b=1.
 4. The method inaccordance with claim 1 wherein the aminoacid Al in the aminoacidalkylphosphonic acid is selected from: D,L-alanine wherein y is 2;L-alanine wherein y is 2; L-phenylalanine wherein y is 2; L-lysinewherein y is in the range from 2 to 4; L-arginine wherein y is in therange from 2 to 6; L-threonine wherein y is 2; L-methionine wherein y is2; L-cysteine wherein y is 2; and L-glutamic acid wherein y is 1 to 2;and from moieties A in the aminoacid phosphonic acid A¹ selected fromC₂-C₁₆ linear hydrocarbon radicals substituted by 1 to 3 NH₂ moieties.5. The method in accordance with claim 4 wherein the amino acidphosphonic acid is selected from: 7-aminoheptanoic acid; 6-aminohexanoicacid; 5-aminopentanoic acid; 4-aminobutyric acid; and β-alanine; wherebyy is 2 in each of such species.
 6. The method in accordance with claim1, wherein the amino acid alkyl phosphonic acid is made in the presenceof not more than 0.4% of hydrohalogenic acid, expressed in relation tothe phosphorous acid component (100%) by reacting: (a) phosphorous acid;(b) an amino acid of formula A¹ or A²; and (c) formaldehyde: in reactantratios of (a):(b) of from 0.05:1 to 2:1; (c):(b) of from 0.05:1 to 5:1;and (c):(a) of from 5:1 to 0.25:1; wherein (a) and (c) stand for thenumber of moles to be used and (b) represents the number of molesmultiplied by the number of N—H functions in the amine, in the presenceof an acid catalyst having a pKa equal or inferior to 3.1, said catalystbeing homogeneous with respect to the reaction medium and being used inreactant ratios as follows: (b):(d) of from 40:1 to 1:5; wherein (d)stands for the number of moles of catalyst multiplied by the number ofavailable protons per mole of catalyst.
 7. The method in accordance withclaim 1, wherein, in addition to the amino acid alkyl phosphonic acid, apolyphosphonic acid is added, said polyphosphonic acid being selectedfrom the group of (a) aminopolyalkylene polyphosphonic acid 5 whereinthe alkylene moiety contains from 1 to 20 carbon atoms; (b)hydroxyalkylene polyphosphonic acids wherein the alkylene moietycontains from 2 to 50 carbon atoms; and (c) phosphono alkanepolycarboxylic acids wherein the alkane moiety is in straight chainconfiguration containing from 3 to 12 carbon atoms; in a weight ratio ofamino acid alkyl phosphonic acid to polyphosphonic acid in the range offrom 98:2 to 25:75.
 8. The method in accordance with claim 1, whereinthe bleaching treatment is conducted at a temperature of from 40 to 90°C. for a period of from 8 minutes to 6 hours.
 9. The method inaccordance with claim 1, wherein the pulp represents from 3 to 30% byweight.
 10. The method in accordance with claim 1 wherein the bleachingagent is an oxidative bleach selected from hydrogen peroxide, oxygen,peroxide reinforced oxygen, chlorine dioxide, ozone and peracids, saidbleaching agent being added at a level from 1% to 6% by weight, saidaqueous medium having a pH of from 8 to
 12. 11. The method in accordancewith claim 10 wherein the oxidative bleaching is conducted at a pH offrom 8-12 regulated with the aid of magnesium hydroxide, silicate bufferand/or Epsom salt.
 12. The method in accordance with claim 10 whereinthe oxidative bleaching is conducted at a pH in the range of from8.5-10.5.
 13. The method in accordance with claim 1, wherein thebleaching agent is a reducing bleaching agent selected from the group ofdithionite, formamidine sulfinic acid and boronhydride.
 14. The methodin accordance with claim 13 wherein the reductive bleaching is conductedat a pH of from 4-6.5.